Process for the production of adiponitrile

ABSTRACT

Process of hydrodimerization of acrylonitrile to adiponitrile by the direct electrolytic route, by passing a direct electrical current through an electrolytic cell having the anode and cathode in contact with the electrolytic medium, which comprises using an initial electrolysis medium consisting essentially of (a) acrylonitrile, (b) water, (c) at least one alkali salt selected from the group consisting of the alkali salts of condensed polyphosphoric acids of the formula

United States Patent [72] Inventors Albert VerheydenSaint-Denis-Westrem; Jean Walravens, Watermael-Boitsiort, both ofBelgium [2]] Appl. No. 830,521

[22] Filed [45] Patented June 4, 1969 Oct. 26, 1971 [73] Assignee UCBSociete Anonyme Salnt-Gelles-lez-Brussels, Belgium [32] Priority June 6,1968 [33] Great Britain [54] PROCESS FOR THE PRODUCTION OF [56]References Cited UNITED STATES PATENTS 3,458,559 7/1969 Holland et al.204/73 Primary Examiner-F. C. Edmundson Attorney-Wenderoth, Lind &Ponack ABSTRACT: Process of hydrodimerization of acrylonitrile toadiponitrile by the direct electrolytic route, by passing a directelectrical current through an electrolytic cell having the anode andcathode in contact with the electrolytic medium, which comprises usingan initial electrolysis medium consisting es sentially of (a)acrylonitrile, (b) water, (c) at least one alkali salt selected from thegroup consisting of the alkali salts of condensed polyphosphoric acidsof the formula n H -,PO,(n-I) H 0 (1) in which n has a value of from 2to I00, and the alkali salts of polymetaphosphoric acids of the formulaII II RII in which n has a value of from 2 to 100, (d) a surface-activesubstance, and (e) at least one acidic salt of an alkali metal and ofapolyacid, the ratio by weight of (e) to (c) being com prised between99.9/0.l and 0/100.

PROCESS FOR THE PRODUCTION OF ADIPONITRILE The present invention isconcerned with a process .for the production of adiponitrile fromacrylonitrile.

It is well know to produce adiponitrile by the hydrodimerization ofacrylonitrile either by operating in the presence of a metal amalgam orby the direct electrolytic route.

For the direct electrolytic hydrodimerization of acrylonitrile to giveadiponitrile, which is what the present invention is concerned with,there are already known several methods. Thus, it has been proposed tocarry out the cathodic hydrodimerization of acrylonitrile in aconcentrated aqueous solution of tetramethyl ammonium toluene sulfonateon a cathode having a hydrogen over voltage greater than that of copper(Belgian Pat. specification Nos. 631,302 and 640,836). Another processconsists in hydrodimerizing acrylonitrile by the electrolysis of ainixture of acrylonitrile and of a small quantity of water saturated byan electrolyte, such as lithium bromide, on a platinum electrode(Belgian Pat. specification No. 649,625).

These known processes present a number of disadvantages: high terminalvoltages because of the low conductivity of the system, formation ofpolymers to the detriment of the selectivity of adiponitrile,consumption of the platinum electrode which greatly increases the costof the product and difficulties of recovery caused by the dissolution ofthe electrolyte in the reaction product.

Subsequently, an improvement was achieved according to which it wassuggested to carry out the electrolytic hydrodimerization ofacrylonitrile emulsified in an alkaline aqueous solution contained in anelectrolysis device, without a diaphragm, furnished with a graphitecathode (French Pat. specification No. 1,401,175), a procedure whichcoped, to a large extent, with the difficulties mentioned above.Nevertheless, the yields of adiponitrile obtained by this process do notreach 75 percent of the theoretical value and, to avoid thesaponification of the nitrile groups by the alkali, the process must becarried out at a low temperature (about C.) which results in theconsumption ofa large amount of energy for the refrigeration of theelectrolytic solution.

However, the inconveniencies of the above-mentioned process have, to alarge extent, been overcome by a recent process (Belgian Pat.specification No. 684,436) in which the electrolysis is carried out inan emulsion in an electrolyte containing incompletely substituted saltsof an alkali metal and of a polyacid, as well as surface-activesubstances. The electrolysis apparatus used is of the type without adiaphragm in which there is used an anode constituted by an iron oxidesupported on metallic iron and possibly containing up to percent ofoxides of silicon and up to 10 percent ofoxides of titanium. Byincompletely substituted salts of an alkali metal and of an inorganic ororganic polyacid, there are intended the sulfates, borates, perborates,phosphates, oxalates and the like of the alkali metals.

The process according to Belgian Pat specifications No. 684,436 providesa remarkable technical advance in that it enables excellent yields to beobtained not only as referred to the acrylonitrile consumed but also asto the amount of electricity supplied to the system. However, a majordisadvantage is that the iron oxide anode is subjected to a veryconsiderable degree of corrosion during the course of the electrolysis.This corrosion necessitates the frequent replacement of the anodes,which represents a very heavy expense to the point of compromising theindustrial application of the invention. Consequently, there is a verygreat interest in preserving substantially all of the advantagesprovided by this process, while reducing the corrosion of the anode to aminimum. lt is this which constitutes the object of the presentinvention.

According to the present invention, we have found that the electrolyteused according to the process of Belgian Pat. specification No. 684,436is responsible for the corrosion of the iron oxide anode and that, ifthe salts of the electrolyte are replaced wholly or partially by thealkali metal salts of polycondensed phosphoric acids, it is possible tomaintain substantially all of the advantages of this process, whilereducing the corrosion of the iron oxide anode to an amount which istechnically acceptable.

The process according to the present invention for the hydrodimerizationof acrylonitrile to adiponitrile by the direct electrolytic route, bypassing a direct electrical current through an electrolytic cell havingthe anode and cathode in contact with the electrolytic medium, comprisesusing an initial electrolysis medium consisting essentially of (a)acrylonitrile, (b) water, (c) at least one alkali salt selected from thegroup consisting of the alkali salts of condensed polyphosphoric acidsof the formula:

nH PO -(nl )H O (l) in which n has a value of from 2-100, and the alkalisalts of polymetaphosphoric acids of the formula n H M in which n has avalue of from 2-100 (d) a surface-active substance and (e) possibly atleast one acidic salt of an alkali metal and ofa polyacid.

By alkali salts of condensed polyphosphoric acids of formula (1), thereare intended the sodium, potassium, lithium, ammonium and quaternaryammonium salts of acids, such as pyrophosphoric acid (H,P O,-),triphosphoric acid (H P O tetraphosphoric acid (H -P 0 polyphosphoricacids containing from 5 to 100 phosphorus atoms, and mixtures thereof.

'By alkali salts of polymetaphosphoric acids of formule (11), there areintended the sodium, potassium, lithium, ammonium and quaternaryammonium salts of acids, such as dimetaphosphoric acid (H P- O),tn'metaphosphoric acid (11 F 0 tetrametaphosphoric acid (H,P.oa).metaphosphoric acids containing from 5 to 100 phosphorus atoms andmixtures thereof.

The alkali salts of the condensed polyphosphoric acids of formula (1)and of the metaphosphoric acids of formula (11) may also be used in theform of mixtures with one another in any desired proportions;furthermore, there can be used the alkali salts of these acids such asare available commercially, for example, under the names of Graham'ssalt, Kurrol salt, sodium hexametaphosphate, SQ salt sold by Monsanto(Na,,P 0 and the like.

By acidic salts of alkali metals and of polyacids, there are intendedthe salts of a polyacid, such as sulfuric acid, boric acid, perboricacid, phosphoric acid, oxalic acid or the like, which is incompletelysubstituted, containing at least one hydrogen cation. Thus, there may bementioned monoand disodium orthophosphates, monoand dipotassiumorthophosphates, sodium hydrogen sulfate, monopotassium oxalate and thelike, as well as mixtures thereof.

As will be shown in the following examples, the salts used according tothe present invention may replace wholly the partially substituted saltsof alkali metals and of polyacids, especially the acidic alkali metalorthophosphates. However, as some of the salts according to the presentinvention are more expensive, especially with regard to the alkaliorthophosphates, and as, on the other hand, their lower ionization,increases the terminal voltage, it is intended, according to the presentinvention, to use mixtures, on the one hand, of acidic salts of alkalimetal and of polyacid and, on the other hand, of salts of acidsaccording to formulas (l) and (11) in which the amount of polyphosphatesaccording to the present invention is sufficient to maintain thecorrosion of the anode at an acceptable level. Indeed, we have,surprisingly, found that the amount of polyphosphate may be relativelylow, without prejudicing the anticorrosive effect. Thus, the ratio byweight between the acidic salts of alkali metal and of polyacid and ofthe polyphosphates used according to the present invention may be99.9/01 to 0/100, advantageously 99/1 to /20, and preferably 95/5 to/15.

The concentration by weight of the polyphosphates (or of the mixture ofpolyphosphates and of acidic salts of alkali metal and of polyacid) inthe aqueous electrolytic solution may'vary from 0.5 percent up to theconcentration corresponding to saturation.

As surface-active substances, there may be used quaternary ammoniumsalts or pyridinium salts, such as acidic bistetraethyl-amrnoniumphosphate, penta-tetraethyl-ammonium tripolyphosphate or acidicbis-methyl-pyridinum phosphate or the like. The concentration of thesesurface-active substances in the aqueous electrolytic solution may varyfrom 0.05 to 5 percent by weight, preferably from 0.2 to 2 percent Apartfrom the polyphosphates (and possibly of the acid salts of an alkalimetal and of a polyacid) and the surface-active substances, the initialelectrolytic solution essentially contains water. However, there mayalso be added a small quantity of a base or of an acid in order tomaintain a definite pH value, this pH value advantageously beingmaintained between 5 and 10, preferably between 8 and 9.

In the course of the electrolysis, a mixture of emulsified acrylonitrileand of the initial aqueous electrolytic solution is circulated throughthe electrolysis apparatus, the volumetric ration between the aqueousphase and the acrylonitrile phase being maintained within the limits of1:1 and 6: l.

This anode is surrounded by two cathodes made of graphite and of thesame dimensions, placed on both sides of the anode at a distance ofl cm.

The supply of current is made by means of steel threaded rods screwed tothe upper part ofeach electrode.

The assembly of the three electrodes is fixed vertically in the beaker.

The apparatus is provided with [.8 liters of the electrolytic solutionto be tested. A temperature of 20 C. is maintained and there is passedthrough a continuous current of 14 amperes. The current density is inthe region of 7 amperes/dmf". Every 24 hours, the electrolysis isinterrupted for the time necessary to remove the anode, rinse it, dryand weigh it. The anode is then replaced and the electrolysis resumedunder the same conditions.

in the following table I, there are set out the results thus obtained byexpressing the rate of corrosion of the anode in mm. loss of thicknessper year.

The first experiment is a comparative one carried out with an TABLE IExperiment number KzEPO 5.6 5.6 5.3 4.8 KsPzOnL 0. 06 0.3 1.14 (EtNhllPOt 1 1 1 1 Hexarnetaphosphatc Phosphate SQ NB P207 Rate ofcorrosion, mmJyear:

irst day 5.31 12.31 1.09 0.48 0. 3.14 0.96 0.51 0.16 O. 2. 0. 96 0. 480.16 0. 2. 53 1.06 0.32 0.03 0. Terminal voltage 5 5 5 5. 6

The temperature during the electrolysis is maintained within the limitsof 0 C., to 40 C., preferably within the region of room temperature.

The linear velocity of circulation of the emulsified mixture is between0.1 and l m./sec.

it is preferred to use an electrolysis apparatus without a diaphragm,having graphite cathodes and magnetite anodes, with or without ametallic support. The current density is l20 amperes/dm. and the voltageis comprised between 4 and volts preferably between 4 and 7 volts.

The process according to the present invention is equally applicable toanodes made of materials other than iron oxide, for example, anodes madeof metallic iron or the like.

In general, the electrolysis is carried out in a manner such that theconversion of the acrylonitrile is -70 percent, preferably -50 percent.Below 20 percent, the economy of the process according to the presentinvention or the production ratio is too poor for industrial use, whilewhen the conversion is increased beyond 70 percent, the selectivity ofadiponitrile is less good.

The process according to the present invention may be carried out aswell discontinuously as continuously.

The following examples are given for the purpose of illustrating thepresent invention:

EXAMPLE 1.

a. Comparative experiments for the corrosion of magnetite anodes.

The apparatus for measuring the rate of corrosion of the anodescomprises a beaker provided with means for cooling and a mechanicalstirrer.

The magnetite anode subjected to the experiments is a square plate ofsteel or of Armco iron, the edges of which are 10 cm. long and thethickness of which is l cm., entirely covered with a coating ofmagnetite with a thickness of about 1 mm., obtained by the superficialoxidation of the metal in water vapor at a temperature of 1,000 C.

electrolyte of the kind used according to Belgian Pat. specification No.684,436. It can be seen that the corrosion of the electrode made of ironoxide brought about by this electrolyte is considerable.

In experiments 2-5, in which the acidic potassium orthophosphate isreplaced by an increasing amount of potassium tripolyphosphate, it isobserved that the corrosion decreases with the increasing amount oftripolyphosphate and that on the fourth day it is not more than l/th ofthe corrosion according to the first experiment. A comparison ofexperiments l and 4 also shows that it is possible to reduce eightyfoldthe rate of corrosion by replacing only 20 percent of the orthophosphateby polyphosphate.

Experiments 6, 7 and 8 show that with other polyphosphates orpolymetaphosphates, there is also obtained, according to the presentinvention, a considerable diminution of the corro- SlOl'l.

Experiments 9, l0 and l 1 point out that at very low doses of sodiumhexametaphosphate the decrease of corrosion is already considerable.

b. Comparative experiments for the corrosion of iron anodes. When themagnetite anode of the experiment carried out as in example 1 (a) isreplaced by an iron anode, the results represented in the followingtable ll are obtained:

TABLE II Experiment number PP mtoenoaen How:

NOTE: The favorable effect of replacing a part of KQIIPOJ by K P Om canclearly been seen from this table.

EXAMPLE 2.

ELECTROLYTIC HYDRODIMERISATION OF ACRYLONITRILE IN PRESENCE OFPOLYPHOSPHATES.

In a semiindustrial electrolytic cell made of polypropylene of thefilter press type comprising 6 compartments, each of which is delimitedby a flat graphite cathode with an effective area of 3.4 dm. and by aflat steel anode covered with a coating of magnetite, which also has aneffective surface area of 3.4 dmF, having a distance between thecathodes and anodes of 5 mm., there is carried out the electrolysis ofan emulsion containing acrylonitrile and the aqueous solution(electrolyte). The composition of this emulsion and the conditions ofelectrolysis are given in the following table III.

In experiment No. l, the composition of the electrolyte is of the typeused in Belgian Pat. specification No. 684,436; it contains, inparticular, acidic dipotassium orthophosphatev In the second experiment,which is according to the present invention, a part of the acidicdipotassium orthophosphate of the first experiment is replaced bypotassium tripolyphosphate.

In the third experiment, according to the present invention, a part ofthe acidic dipotassium orthophosphate of the first experiment isreplaced by sodium hexametaphosphate.

In the fourth experiment, according to the present invention, the wholeof the dipotassium orthophosphate is replaced by potassiumtripolyphosphate.

In the four experiments, the electrolysis device functions in acontinuous manner, with a constant supply of acrylonitrile and of water(to compensate for the electrolytic decomposition of this latter) and acontinuous removal, by decantation, of an organic phase containingunchanged acrylonitrile,

adiponitrile, propionitrile and the products of hydro oligomerisation.The pH value is permanently controlled and maintained at 8.4.

In the following table:

AN=Acrylonitrile. ADN=Adiponitrile. PN=Propionitrile. Efi.ADNlAN=Percent of the number of moles of ADN formed referred to thenumber of moles of AN supplied.

Efi. PNlAN=Percent of the number of moles of PN formed, referred to thenumber of moles of AN supplied.

Efi. Hydr. Fraction in percent otAN supplied which is transformed intohydrooligomers.

Yield ADN/AN=Quotient in percent of the efficiency of ADN by theconversion of AN.

Yield ADN/Elec.=Ratio in percent between the number of moles of ADNformed and the number of taradays provided to the cell in the course ofthe electrol sis.

TABLE III Experiment Number Duration of electrolysis (hrs.) 227 215 117.5 83 Percent composition of the initial aqueous phase emulsion:

3O 93. 46 93. 30 93. 30 93. 5 KIHPO4- 5. 57 5. 36 5. 36 KP30m 0 O. 33 05. 54: Na hexametaphosphat 0 0 0. 33 0 (EtlN)JHPOl 0. 96 O. 96 0. 97 O 4)5 a 1o 0 0 0 O. 96 Initial organic phase AN AN AN AN Ratio by vol. aq.ph./org. ph. 2 2 2 2 Current intensity (amps) 157 157 158 158 Currentdensity (amp/dm. c. 7.9 7.9 7. 9 7 Rate of circulation of emulsion (dm./

sec. 3 3 3 3 Supply of AN (g./hr.) 634 634 634 634 Supply of water(g./hr.) 72 78. 5 78.5 78. 5 Unchanged AN (percent) c. 51. 2 51.7 51. 952. 9 Efliciency ADN/AN (percent) 39. 0 36.1 37.4 35. 7 Efiiciency PN/AN(percent) 3. 6 4. 4 4. 35 3. 2 Efficiency HydrJAN (percent) 4. 8 7.8 8.2 Yield ADN/AN (percent) 80.0 74. 7 77.8 75. 7 Yield ADN/elec. current(percent) 77. 8 70. 5 73.0 73. 6 Rate of corrosion of the magnetite(IDJIL/ year) 2. 2 0. 6 0. 3 1. 2 Terminal voltage 5. 8 6. 3 6. 3 6. 1Specific conductivity of the emulsion (Q' CBL' 18. 10 l4. l0 l2. l0

The table shows that the yield of adiponitrile referred to the suppliedacrylonitrile is appreciably maintained and that the yield ofadiponitrile with reference to the electric current is subjected to aslight diminution. However, this small loss in yield is largelycompensated by the diminution of the speed of corrosion of the anodewhich, compared with that found in experiment No. 1, is only about onehalf in experiment No. 4, is less than one third in experiment No. 2 andis less than one seventh in experiment No. 3.

EXAMPLE 3.

The experiment was carried out in a tubular electrolytic cellconstituted by a cylindrical anode made of molten magnetite surroundedby a graphite tube which functions both as cathode and as container. Theanode has a diameter of 6 cm. and an effective length of 61 cm., i.e., asurface of 11.3 dm. dm.? The cathode has an interior diameter of7 cm.,i.e.. an effective surface of l3.2 dm.? The distance between theelectrodes is 0.5 cm. The cell proper is completed by a tubing systemwith pumps, hydrocyclone and filter allowing the circulation of thereaction medium between the electrodes and the collection of the solidproducts resulting from the corrosion of the anode (phosphate+ironhydroxide). Cooling is effected by circulating brine in a double jacketsurrounding the graphite tube.

The working conditions and the obtained results are given in thefollowing table IV:

TABLE IV Experiment Number Percent composition of the emulsion:

Initial aqueous phase:

93. 3 93. 3 5. 7 5. 4 0 0. 3 1. 0 I. 0 Initial organic phase AN AN Ratioby vol. an. ph./org. ph. 2 2 Current intensity (amps) 90 90 Anodlccurrent density (amp/dm. 7. 95 7. 95 Cathodic current density (amp/dm.6. 6. 80 Rate or circulation (dm./sec.) 3 3 Supply of AN (g./hr.) 372393 Supply of water (g./hr.) 47 47 Unchanged AN (percent).. 53. 4 58. 3Efficiency ADN/AN (percent)... 38. 8 34.6 Etiicicncy PN/AN (percent) 3.73.0 Etficiency Hydr./AN (percent) 4.1 4.1 Yield ADN/AN (percent) 83. 282. 9 Yield ADN/elcc. current (percent) 80. 2 75. 5 Rate of corrosion oftho magnetite anodes (mm./

year) 15. 4 0.3 Terminal voltage 5. 6 5. Q Specific conductivity of theemulsion (tr cmr -2.10' -2. 10-- This table shows that on replacingabout 5% of K vHPO, by sodium hexametaphosphatc, practically the sameyield of adiponitrile is maintained whereas the corrosion ofthe anode isfiftyfold diminished.

We claim:

1. Process of hydrodimerization of acrylonitrile to adiponitrile by thedirect electrolytic route, by passing a direct electrical currentthrough an electrolytic cell having the anode and cathode in contactwith the electrolytic medium, which comprises using an initialelectrolysis medium consisting essentially of (a) acrylonitrile, (b)water, (c) at least one alkali salt selected from the group consistingof the alkali salts of condensed polyphosphoric acids of the formula inwhich n has a value of from 2 to I00, and the alkali salts ofpolymetaphosphoric acids of the formula n n fln in which n has a valueof from 2 to I00, (d) a surface-active substance, and (e) at least oneacidic salt of an alkali metal and ofa polyacid, the ratio by weight of(e) to (c) being comprised between 99.9/0.l and 0/l00.

2. Process as claimed in claim I, in which the ratio by weight ot(e) to(c) is comprised between 99/1 and 80/20.

3. Process as claimed in claim I, in which the ratio by weight of (e) to(c) is comprised between 95/5 and /15.

4. Process as claimed in claim I, in which the concentration 5. Processas c laimed in claim 1, in which the salts of formu la (1) are selectedfrom the group consisting of sodium, potassium, lithium, ammonium andquaternary ammonium salts of pyrophosphoric, triphosphoric,tetraphosphoric and polyphosphoric acids containing from 5 to 100phosphorous atoms and mixtures thereof.

6. Process as claimed in claim 1, in which the salts offormula (II) areselected from the group consisting of sodium, potassium, lithium,ammonium and quaternary ammonium salts of dimetaphosphoric,trimetaphosphoric, tetrametaphosphoric acids and metaphosphoric acidshaving 5 to 100 phosphorous atoms and mixtures thereof.

7. Process as claimed in claim 1, in which the salts of formula (1) areused in mixture with salts of formula (11).

8. Process as claimed in claim 1, in which the surface-active substanceis selected from the group consisting of quaternary ammonium andpyridinium salts.

9. Process as claimed In claim 1, in which the surface-active substanceis selected from the group consisting of acidic bistetraethyl-ammoniumphosphate, penta-tetraethyl-ammonium tripolyphosphate and acidicbis-methylpyridinium phosphate.

10. Process as claimed in claim 1, in which the concentration of thesurface-active substance in the aqueous electrolytic solution iscomprised between 0.05 and 5 percent by weight.

11. Process as claimed in claim 1, in which the concentration of thesurface-active substance in the aqueous electrolytic solution iscomprised between 0.2 and 2 percent by weight.

12. Process as claimed in claim 1, in which the acidic salt of an alkalimetal and of a polyacid is selected from the group consisting of acidicsalts of sulfuric, boric, perboric, phosphoric and oxalic acids, whichare incompletely substituted, containing at least one hydrogen cation.

13. Process as claimed in claim 1, in which the acidic salts of analkali metal and of a polyacid is selected from the group consisting ofacidic sodium and potassium salts of orthophosphoric acid, sodiumhydrogen sulfate and 8 monopotassium oxalate.

14. Process as claimed in claim 1, in which an amount of base or acid isadded to the water so as to maintain a pH value comprised between 5 and10.

15. Process as claimed in claim 1, in which an amount of base or acid isadded to the water so as to maintain a pH value comprised between 8 and9.

16. Process as claimed in claim 1, in which, in the course ofelectrolysis, the volumetric ratio of the aqueous electrolytic solutionto acrylonitrile is maintained between 1: 1 and 6:1

17. Process as claimed in claim 1, in which, in the course ofelectrolysis, the temperature is maintained within the limits of 0 C.and 40 C.

18. Process as claimed in claim 1, in which, in the course ofelectrolysis, the temperature is maintained within the region of roomtemperature.

19. Process as claimed in claim 1, in which the linear velocity ofcirculation of the emulsified mixture of the aqueous electrolyticsolution and acrylonitrile in the electrolysis apparatus is comprisedbetween 0.1 and 1 m./sec.

20. Process as claimed in claim 1, in which electrolysis is carried outin an electrolysis apparatus without diaphragm having graphite cathodesand magnetite or iron anodes.

21. Process as claimed in claim 1, in which the current density iscomprised between 1 and 20 arnperes/dm. and the tension is comprisedbetween 4 and 10 volts.

22. Process as claimed in claim 1, in which the current density iscomprised between 1 and 20 amperes/dm. 1 and the tension is comprisedbetween 4 and 10 volts.

23. Process as claimed in claim 1, in which the electrolysis is carriedout in a manner such that the conversion of acrylonitrile is comprisedbetween 20 and percent.

24. Process as claimed in claim Lin which the electrolysis is carriedout in a manner such that the conversion of acrylonitriie is comprisedbetween 40 and 50 percent.

25. Process as claimed in claim 1, in which the electrolysis is carriedout discontinuously.

26. Process as claimed in claim 1, in which the electrolysis is carriedout continuously.

2. Process as claimed in claim 1, in which the ratio by weight of (e) to(c) is comprised between 99/1 and 80/20.
 3. Process as claimed in claim1, in which the ratio by weight of (e) to (c) is comprised between 95/5and 85/15.
 4. Process as claimed in claim 1, in which the concentrationof the mixture (e)+(c) in the aqueous electrolytic solution is comprisedbetween 0.5 percent by weight and the concentration corresponding tosaturation.
 5. Process as claimed in claim 1, in which the salts offormula (I) are selected from the group consisting of sodium, potassium,lithium, ammonium and quaternary ammonium salts of pyrophosphoric,triphosphoric, tetraphosphoric and polyphosphoric acids containing from5 to 100 phosphorous atoms and mixtures thereof.
 6. Process as claimedin claim 1, in which the salts of formula (II) are selected from thegroup consisting of sodium, potassium, lithium, ammonium and quaternaryammonium salts of dimetaphosphoric, trimetaphosphoric,tetrametaphosphoric acids and metaphosphoric acids having 5 to 100phosphorous atoms and mixtures thereof.
 7. Process as claimed in claim1, in which the salts of formula (I) are used in mixture with salts offormula (II).
 8. Process as claimed in claim 1, in which thesurface-active substance is selected from the group consisting ofquaternary ammonium and pyridinium salts.
 9. Process as claimed in claim1, in which the surface-active substance is selected from the groupconsisting of acidic bis-tetraethyl-ammonium phosphate,penta-tetraethyl-ammonium tripolyphosphate and acidicbis-methylpyridinium phosphate.
 10. Process as claimed in claim 1, inwhich the concentration of the surface-active substance in the aqueouselectrolytic solution is comprised between 0.05 and 5 percent by weight.11. Process as claimed in claim 1, in which the concentration of thesurface-active substance in the aqueous electrolytic solution iscomprised between 0.2 and 2 percent by weight.
 12. Process as claimed inclaim 1, in which the acidic salt of an alkali metal and of a polyacidis selected from the group consisting of acidic salts of sulfuric,boric, perboric, phosphoric and oxalic acids, which are incompletelysubstituted, containing at least one hydrogen cation.
 13. Process asclaimed in claim 1, in which the acidic salts of an alkali metal and ofa polyacid is selected from the group consisting of acidic sodium andpotassium salts of orthophosphoric acid, sodium hydrogen sulfate andmonopotassium oxalate.
 14. Process as claimed in claim 1, in which anamount of base or acid is added to the water so as to maintain a pHvalue comprised between 5 and
 10. 15. Process as claimed in claim 1, inwhich an amount of base or acid is added to the water so as to maintaina pH value comprised between 8 and
 9. 16. Process as claimed in claim 1,in which, in the course of electrolysis, the volumetric ratio of theaqueous electrolytic solution to acrylonitrile is maintained between 1:1and 6:1.
 17. Process as claimed in claim 1, in which, in the course ofelectrolysis, the temperature is maintained within the limits of 0* C.and 40* C.
 18. Process as claimed in claim 1, in which, in the course ofelectrolysis, the temperature is maintained within the region of roomtemperature.
 19. Process as claimed in claim 1, in which the linearvelocity of circulation of the emulsified mixture of the aqueouselectrolytic solution and acrylonitrile in the electrolysis apparatus iscomprised between 0.1 and 1 m./sec.
 20. Process as claimed in claim 1,in which electrolysis is carried out in an electrolysis appAratuswithout diaphragm having graphite cathodes and magnetite or iron anodes.21. Process as claimed in claim 1, in which the current density iscomprised between 1 and 20 amperes/dm.2 and the tension is comprisedbetween 4 and 10 volts.
 22. Process as claimed in claim 1, in which thecurrent density is comprised between 1 and 20 amperes/dm.2 and thetension is comprised between 4 and 10 volts.
 23. Process as claimed inclaim 1, in which the electrolysis is carried out in a manner such thatthe conversion of acrylonitrile is comprised between 20 and 70 percent.24. Process as claimed in claim 1, in which the electrolysis is carriedout in a manner such that the conversion of acrylonitrile is comprisedbetween 40 and 50 percent.
 25. Process as claimed in claim 1, in whichthe electrolysis is carried out discontinuously.
 26. Process as claimedin claim 1, in which the electrolysis is carried out continuously.